A tissue anchor includes a tissue-engaging element, a head, and a protrusion. The tissue-engaging element has a sharpened distal tip, and is configured to be driven into tissue of a subject. The head is coupled to a proximal end of the tissue-engaging element, and includes a driver interface that is configured to be reversibly engaged by an anchor driver. The protrusion protrudes away from the head such that driving the tissue-engaging element into the tissue presses the protrusion against the tissue, and is configured to move with respect to the head automatically in response to being pressed against the tissue. Other embodiments are also described.